A dam, a canal or a pumping station is usually provided with a gate equipped with a gatage indicator for measuring the degree of gate opening or displacement of the top of the gate. The measured value is transmitted to a display panel in the control room, wherein water discharge is controlled under monitoring. For some gates, the gate opening is converted into flow rate data for proper operations. Accordingly, precision of actual displacement is very essential to gate operation.
However, a conventional gatage indicator usually uses a circular analog meter, which is driven by mechanical gears to simultaneously drive a potentiometer to directly convert the actual displacement detected into an analog signal according to a potentiometric ratio. Thereby measured values lack accuracy, such that, for example, the displayed data converted from the analog signal in the control room changes irregularly even if the gate is in a still state. Further advancement on the conventional detecting device is needed.
Therefore, the present inventor has disclosed a Taiwanese Utility Model Patent No. 140156 entitled "Cable extension type gatage transducer". The above mentioned shortcomings of the conventional measuring devices are improved. However, its disadvantages of higher production costs and larger occupied space should be further improved. Moreover, if an extended cable retracts at an irregular speed, the simply integrating of the drum and detecting roller will get the retracted cable disorderly winding on the drum to cause inaccurate displacement display. Besides, if extended but accidentally loosened, the cable will retract at high acceleration and release from the groove of the drum. As a result, both precision measurement and operation performance will be affected.
In addition, U.S. Pat. No. 5,236,144 "Cable Extension Linear Position Transducer" has disclosed that the displacement of an extended cable is detected by using a cable-driven drum to synchronously drive a potentiometer so that the detected displacement is directly converted into an analog signal according to potentiometric ratio. Nevertheless, no accessory is provided for guiding the extended cable to the orderly wound state when retracting at an irregular speed. Moreover, the lower accuracy occurs during the aforesaid signal conversion. In other designs, optical encoders which offer better precision are used, but these devices do not provide proper rolling up or, with the rolling wheel and the sensor wheel placed next to each other, take up a large volume. Moreover, the drawback that the breakdown easily occurs when the extended cable releases by accident or retracts at high acceleration should be improved.
In order to facilitate installation and to avoid breakdown, the present invention provides an integral structure comprising a spool, a spiral spring and a magnet-detecting element to secure greater reliability and accuracy on the actual displacement of the monitored object which is attached to the extension cable.